1. Field of the Invention
The present invention relates to a leakage resistance detection device for an on-board high voltage device, which is connectable to an on-board high voltage device including an on-board high voltage DC power source and a high voltage electric load to be fed and driven by the high voltage DC power source and which measures a leakage resistance with respect to a vehicle body as typified by equivalent leakage resistances on the positive and negative potential sides of the on-board high voltage device, so as to inform the abnormality when the leakage resistance reduces, and also relates to a leakage resistance detection method for the on-board high voltage device.
2. Description of the Related Art
In general, an electric car, a hybrid electric car, and other such vehicles use a combination of a DC 12-V low voltage battery and a high voltage battery such as a DC 400-V battery pack, for example. A negative terminal of the low voltage battery is connected to a vehicle body, but the high voltage battery and a high voltage electric load to be fed and driven by the high voltage battery are mounted on the vehicle body while being entirely insulated from the vehicle body. In this type of vehicle, a leakage resistance detection device, which is fed and driven by the low voltage battery, measures an insulation resistance (having the same meaning as the leakage resistance) of the whole high voltage device, to thereby detect the presence or absence of ground abnormality.
For example, a ground detection apparatus for an electric vehicle described in Japanese Patent Application Laid-open No. 2002-209331 is a ground detection apparatus for an electric car that includes a high voltage DC power source electrically insulated from the vehicle body and a three-phase AC motor driven by a DC voltage from the high voltage DC power source. The ground detection apparatus includes a microcomputer that supplies a ground detection signal representing a square wave to the high voltage DC power source via a detection resistor and a coupling capacitor. The microcomputer detects a voltage amplitude value at a ground detection point corresponding to a connection point between the detection resistor and the coupling capacitor. Based on a preset relationship between a voltage amplitude value and an insulation resistance value, the microcomputer converts the detected voltage amplitude value into an insulation resistance value, and compares the converted insulation resistance value with a preset ground determination threshold, to thereby detect the level of degradation in insulation resistance of the high voltage DC power source. Thus, the circuit configuration can be simplified, and the level of reduction in insulation resistance with respect to the vehicle body can be detected with high accuracy.
First, the ground detection apparatus of Japanese Patent Application Laid-open No. 2002-209331 is described in detail with reference to FIGS. 28 and 29. The name of each element is replaced by the name in the present invention to be described later. Referring to FIG. 28 which is an overall configuration diagram of the conventional apparatus, a leakage resistance detection device 50 includes an arithmetic control circuit 20 mainly configured by a microprocessor, and a repetitive signal output circuit 30 and a monitoring signal processing circuit 40.
An on-board high voltage device 60 contains a high voltage electric load (not shown) including a high voltage DC power source 61. The on-board high voltage device 60 is mounted on a vehicle body 11 while being insulated therefrom, but has equivalent leakage resistances R1 and R2. A coupling capacitor 51 has one terminal B connected to, for example, a negative terminal of the on-board high voltage device 60 and another terminal A connected to an output terminal of the repetitive signal output circuit 30.
The arithmetic control circuit 20 generates a repetitive command signal PLS, which is a pulse train signal in which an “H” period T1 and an “L” period T2 satisfy T1=T2 (=half cycle T). The repetitive signal output circuit 30 includes a charge/discharge switching element 39 and a series resistor R0. The charge/discharge switching element 39 is formed of a pair of upper and lower transistors, one of which becomes conductive in response to the repetitive command signal PLS. When the output logic level of the repetitive command signal PLS is “H”, the coupling capacitor 51 is supplied with a charge current from a predetermined control power supply voltage Vcc via the upper transistor (not shown) and the series resistor R0. At this time, the lower transistor is opened.
On the other hand, when the output logic level of the repetitive command signal PLS is “L”, a discharge current of the coupling capacitor 51 flows via the lower transistor (not shown) and the series resistor R0. At this time, the upper transistor is opened. The monitoring signal processing circuit 40 inputs the value of a monitoring voltage Vx, which is the potential between the another terminal A of the coupling capacitor 51 and the vehicle body 11, the another terminal A being a measurement point, to the arithmetic control circuit 20 via a noise filter (not shown) and an operational amplifier 49 as an analog signal voltage ANL.
In the first period T1=T in which the output logic level of the repetitive command signal PLS is “H”, the value of the analog signal voltage ANL increases gradually from an initial voltage Vx1 to an end voltage Vx2. In the second period T2=T in which the output logic level of the repetitive command signal PLS is “L”, the value of the analog signal voltage ANL decreases gradually from the initial voltage Vx2 to the end voltage Vx1. Note that, in the case where the leakage resistance fluctuates due to voltage fluctuations of the high voltage DC power source 61 or due to ON/OFF of a power supply switch for load driving, the voltage of the coupling capacitor 51 at the measurement point A becomes a value equal to or lower than 0 volts or equal to or larger than the control power supply voltage Vcc relative to the vehicle body potential, thus deviating from a proper voltage range.
When ground abnormality has occurred in the on-board high voltage device 60, bypass diodes 815 and 816 and bypass diodes 817 and 818 feed back the electric charges stored in the coupling capacitor 51 to the vehicle body 11 via an output terminal and a negative terminal of a constant voltage control power source (not shown) that generates the control power supply voltage Vcc, irrespective of the operating state of the charge/discharge switching element 39. Thus, the value of the monitoring voltage Vx can be returned to the proper voltage range of 0 to Vcc quickly. However, if a leakage resistance Rx is too small, the bypass diodes 817 and 818 at the upstream stage may be disconnected by an overcurrent. In this case, a limited current is fed back via the series resistor R0 and the bypass diodes 815 and 816.
In the state in which the value of the equivalent leakage resistance R1 on the positive potential side and the value of the leakage resistance R2 on the negative potential side are equal to each other and in which no charge/discharge current flows through the coupling capacitor 51, the potential at a connection point B relative to the vehicle body is −Vh/2 as compared to a voltage Vh of the high voltage DC power source 61. When the equivalent leakage resistance R1 is short-circuited, the potential at the connection point B relative to the vehicle body becomes −Vh. When the equivalent leakage resistance R2 is short-circuited, the potential at the connection point B relative to the vehicle body becomes 0. Such potential fluctuations cause a charge/discharge current to flow through the coupling capacitor 51, with the result that the potential at the measurement point A significantly fluctuates, and transiently, deviates outside the proper range of 0 to Vcc.
In FIG. 29 as a characteristics chart of the conventional apparatus configured as illustrated in FIG. 28, a leakage resistance coefficient β on the horizontal axis is a ratio between the value of a parallel combined resistance of the equivalent leakage resistances R1 and R2, namely the value of the leakage resistance Rx=R1×R2/(R1+R2), and the series resistance R0. The series resistance R0 is a known constant, and hence the value of the leakage resistance coefficient β=Rx/R0 is proportional to the leakage resistance Rx. A threshold voltage coefficient γ on the vertical axis is a ratio between a deviation voltage Vx2−Vx1 between the end voltage Vx2 and the initial voltage Vx1 described above, and the control power supply voltage Vcc. The control power supply voltage Vcc is a known constant, and hence the value of the threshold voltage coefficient γ=(Vx2−Vx1)/Vcc is proportional to the measured value of the deviation voltage Vx2−Vx1.
A plurality of characteristics curves of FIG. 29 use a reference time coefficient α0 as a parameter. The reference time coefficient α0 is a ratio between the half cycle T of the repetitive command signal PLS and a charge/discharge time constant τ=(R0+Rx)×C=R0×C with respect to the coupling capacitor 51 when the value of the leakage resistance Rx is zero. Both the half cycle T and the charge/discharge time constant τ are known constants, and hence the reference time coefficient α0=T/(R0×C) is a known constant.
For example, in the case where the series resistance R0 is selectively designed to have the same value as a limit leakage resistance Rx0 which is a permissible lower limit value of the leakage resistance Rx, an interest is taken in the value of the threshold voltage coefficient γ when the leakage resistance coefficient β is 1. If the reference time coefficient α0 is selectively designed to be 0.5, the threshold voltage coefficient γ at a point of interest P1 is 0.56.
Therefore, in the case where the control power supply voltage Vcc is 5 V, for example, if the deviation voltage Vx2−Vx1 exceeds 5×0.56=2.8 V at the end of the “H” period of the repetitive command signal PLS, the leakage resistance Rx is in a safe range exceeding the limit leakage resistance Rx0, and if the deviation voltage Vx2−Vx1 falls below 2.8 V, the leakage resistance Rx falls below the limit leakage resistance Rx0 and is in a dangerous range. Note that, the characteristics chart of FIG. 29 is based on Expression III shown at the top of FIG. 29, and Expression III is satisfied in a stable state in which the value of the monitoring voltage Vx falls within the proper range of 0 to Vcc.
The leakage detection apparatus of Japanese Patent Application Laid-open No. 2002-209331 calculates, based on the characteristics chart of FIG. 29, the deviation voltage Vx2−Vx1 between the initial voltage Vx1 and the end voltage Vx2 at the end of the “H” period (or at the end of the “L” period) of the repetitive command signal PLS, and reads out the value of the leakage resistance coefficient β corresponding to the deviation voltage, to thereby detect the current value of the leakage resistance Rx.
The first problem of this method is that the leakage resistance Rx is calculated at every end of the generated pulse of the repetitive command signal PLS (at every logic change) and hence, even if the deviation voltage is sufficiently high and the leakage resistance is normal at the timing before the end of the generated pulse, ground abnormality determination cannot be made in the pulse generation period. The second problem of this method is that, as is apparent from FIG. 29, the characteristics curves for the reference time coefficient α0 exceeding 1.54 are concave at the center, and hence even the same threshold voltage coefficient γ has two solutions of the leakage resistance coefficient β and a correct solution cannot be obtained.
The reason is that the expression indicates that the value of the threshold voltage coefficient γ converges to 1 when the leakage resistance coefficient β is infinite, and the value of the threshold voltage coefficient γ converges to 1 along with the increase in reference time coefficient α0 when the leakage resistance coefficient β is 0, and hence the threshold voltage coefficient γ takes a value in the range of 0 to 1. Note that, when the leakage resistance coefficient β is 0, the value of the threshold voltage coefficient γ decreases along with the decrease in reference time coefficient α0, and γ converses to 0 as α0 approaches 0.
Therefore, in order to avoid the above-mentioned binary problem, it is necessary that the reference time coefficient α0 to be used take a value of 1.0 or less, and it is necessary that the half cycle T of the pulse be a high frequency pulse whose reference time constant τ0 is equal to or smaller than the product of the series resistance R0 and the electrostatic capacitance C of the coupling capacitor 51 (=R0×C). When the repetitive command signal PLS is not a high frequency pulse, there is another problem in that the detection accuracy of the monitoring voltage deteriorates because an abrupt change occurs between the monitoring voltage at the end of the previous cycle and the monitoring voltage at the end of the current cycle.
Therefore, in the case where the repetitive command signal PLS is a high frequency signal, even after the monitoring voltage Vx deviates from the proper range of 0 to Vcc due to an abrupt change in leakage resistance Rx and then returns to the proper range again by the bypass diodes 815 to 818, it is necessary to wait for a large number of operations of the repetitive command signal PLS until a stable initial voltage Vx1 and a stable end voltage Vx2 are obtained. Thus, the ground abnormality determination cannot be made immediately. This is the above-mentioned second problem.
This problem is caused because the coupling capacitor 51 is alternately charged and discharged by the repetitive command signal PLS and hence the next charge starts before the completion of the previous discharge, and the monitoring voltage Vx does not increase or decrease monotonously. The repetitive command signal PLS shows a smaller change per one cycle as the frequency becomes higher and the cycle becomes shorter. The detection accuracy is therefore improved by increasing the frequency, but the response deteriorates.
The third problem of the above-mentioned method relates to the above-mentioned second problem and is that, in a transient delay period from when the monitoring voltage Vx returns to the proper range after the abrupt change in leakage resistance Rx to when a plurality of repetitive command signals PLS are operated to enable the measurement of the leakage resistance Rx, the measured value of the deviation voltage Vx2−Vx1 is smaller than that in the stable state used as the determination condition. Therefore, the obtained leakage resistance coefficient β is a small value, and it may be erroneously determined that ground abnormality has occurred. Thus, a standby time for avoiding such erroneous determination becomes longer.